1. Field of the Invention
The present invention relates to a novel naphthalene compound and a liquid crystal composition containing the same. More specifically, the present invention relates to a novel naphthalene compound having an optically active group and a liquid crystal composition containing the same.
2. Description of the Related Art
In the prior art, CRT devices are most frequently used as display devices for OA instruments, and in the field of instruments having display devices such as OA instruments, there is an increasing demand for a miniaturization and reduction of the weight of these instruments, or an enlargement of the picture area and a reduced thickness of the picture unit of the display devices. Accordingly, to meet the respective usages and demands the CRT devices of the prior art have been replaced by various newly developed display devices, for example, liquid crystal display, plasma display, LED display, EL display, and ECD display devices.
Among these display devices, the liquid crystal display device sends electric signals to a switching element basically by using a liquid crystal compound to change the state of the liquid crystal compound in the switching element, to thereby control the shielding and transmission of light to elicit electric signals, and is practically applied not only as a display device for the OA instruments as mentioned above, but also as a display device for digital watches or portable games, etc., and further, is now used as a display device for moving pictures, such as for a small television receiver.
In the display device using a liquid orystal compound as described above, various drive systems are known, for example, the TN (twist nematic) mode is used as the drive system for current liquid crystal display devices. The TN mode is a system which obtains a display by utilizing the dielectric anisotropy of the molecules in the nematic phase of the liquid crystal compound, whereby the display device is driven by the energy in proportion to the square of the electrical field externally applied (f.alpha.E.sup.2).
In switching elements employing this system, however, to change the displayed picture the position of the molecules of the liquid crystal compound must be changed, and therefore, the drive time is prolonged and a problem arises in that the voltage required for changing the molecule position of the liquid crystal compound, namely the consumption power, becomes greater. In such a switching element, since the switching threshold value characteristic is poor, when switching actuation is attempted by changing the molecular position at a high speed, a leak voltage occurs even at non-display picture portions, whereby the display contrast is often remarkably lowered.
Accordingly, the display system using the TN mode of the prior art is not always an appropriate display device system for displaying moving pictures, particularly as a display device having a large picture area or for a miniature digital television receiver.
A display device utilizing STN (super twist nematic) having an improved switching threshold value characteristic as the TN mode is also used, and by utilizing the STN mode, the switching threshold value characteristic can be improved, and thus the contrast of the display device can be improved. Nevertheless, this method is the same as the TN mode in that it utilizes a dielectric anisotropy, and therefore, the switching time is prolonged and the same tendency as that of the display device utilizinq the TN mode is exhibited when it is used as a display device for moving pictures, particularly a display device having a large picture area or for a miniature digital television receiver.
Conversely, the organic compound synthesized by R. B. Meyer et al, in 1975 exhibited a ferroelectric property, and further, in 1980 R. B. Meyer suggested the use of a ferroelectric liquid crystal compound as the optical switching element, namely the display device, by filling these ferroelectric liquid crystal compounds in a cell with a small gap.
The switching element using the ferroelectric liquid crystal compound as described above, different from the switching element utilizing the TN mode or the STN mode, functions as a switching element only by changing the alignment direction of the molecules of the liquid crystal compound, whereby the switching time is greatly reduced. Further, since the value of Ps.times.E given by the spontaneous polarization (Ps) possessed by the ferroelectric liquid crystal compound and the electrical field intensity (E) is the effective energy strength for changing the direction of alignment of the molecules of the liquid crystal compound, the consumption power is also greatly reduced. Such a ferroelectric liquid crystal compound has two stable states, depending on the direction of the applied electrical field, namely has bistability, and a good switching threshold characteristic, and therefore is particularly suitable as a display device for moving pictures.
When such a ferroelectric liquid crystal compound is used for the optical switching element, the ferroelectric liquid crystal compound must have characteristics such as an actuation temperature range near normal temperature, a wide actuation temperature width, and a high switching speed and switching threshold value voltage within a required range. Especially, the actuation temperature range is very important when using a ferroelectric liquid crystal compound, and the actuation temperature range does not coincide with the service temperature range in many ferroelectric liquid crystal compounds, although the other characteristics may be very good (see, for example, R. B. Meyer et al, J. de Phs., vol. 36 L-p.69, 1975).
Further, in the pre-text of the llth Liquid Crystal Discussion, p. 16 (1985) reported by Mr. Masaaki Taguchi and Mr. Takamasa Harada or in Japanese Unexamined Patent Publication (Kokai) No. 62-10045, for example, ferroelectric liquid crystal compounds having a naphthalene ring are disclosed. The liquid crystal compounds disclosed have a relatively higher practical applicability from the view point of the actuation temperature range, but they do not satisfy all of the other characteristic requirements for a ferroelectric liquid crystal compound, and there is still room for an improvement thereof.